Cathode for discharge tubes



July 18, 1944; MCHE 2,353,635

CATHODE FOR DISCHARGE TUBES Filed Feb. 11, 1941 f/v/ss/on/ Name/41. 4N0Mama/c Beer/ass.

lnvehfror John. O. Aicher,

' by Q His ATTOT'HQH.

Patented July 18, 1944 2,353,635 CATHODE FOR. DISCHARGE TUBES John 0.Aicher, Cleveland Heights, Ohio, assign'o r" A V to General ElectricCompany, a corporation of New York Application February 11, 1941,,Serial No. 378,347-

1 Claim. (Cl. 25027.5)

My invention relates to gaseous discharge devices and more particularlyto an improved-cathode construction whereby radio interference createdby such devices is materially decreased.

The use of gaseous discharge lamps, especially those of the low-pressuretype, while being Satis-f factory in general, has been accompanied by anobjectionable tendency to create disturbances interfering with radios intheir vicinity. These disturbances are particularly prevalent in lampsof the alternating current type which is, of course, the type mostwidely used. While the nature of the phenomenon has not yet been fullyexplained,

it is known that it centers around the cathode.

It may be the result of some sort of random movement of the arc on thecathode surface, or

erratic behaviour of the particles of the cathode coating under theinfluence of the various electric and magnetic fields to which they aresubjected. There is also good evidence to indicate that the disturbancemay arise from the alternate charging and discharging of oxide particleswhich may become wholly or partially insulated from the main cathodebody. Such oxide particles may arise from the oxidation of the tungstencore either by water vapor in the tube, or

by oxygen from the oxides in the coating used es the electron-emittingmaterial. They may even be formed directly of the oxides of the coating.Particles of this nature may behave as minute condensers and the processof discharging them when once chaged may, of course, create radiofrequency currents in the well-known manner. On the other hand, it maybe that in their charged condition they sufiiciently disturb the,electrical field in the vicinity of the cathode to induce erraticvariations in the arc current. Oscillographic records indicate that thedisturbances generally occur at the beginning and end of each half cycleof alternating current, i. e., when the current strength is low.

I have found that the disturbances can be largely eliminated by addingto the cathode a coating of finely divided powder of metal or otherconducting material. This may be added either by mixture with theregular activating coating, or as a separate coating with a suitablebinder. Apparently such conducting particles provide a good conductingsurface over the whole cathode, which conducting surface operates toprevent the formation of charges on the insulated particles alreadymentioned.

It is accordingly an object of my invention to provide a gaseousdischarge lamp which is relatively free of the disturbances causingradio interference. 1 l i It is a further object of the invention toprovidean' improved cathode for discharge lamps whereby radiointerference from said lamps is largely eliminated.

It is still another object of the invention-to provide a cathode for theaforementioned pur: pose which is relatively simple in construction andeconomical in manufacture. Further objects and advantages of theinvert-.-- tion will be apparent from a consideration of the followingdetailed description, and the accompanying drawing which shows a cathodeconstructed in accordance with my invention.

In preparing my improved cathode, I add to any of the conventionalactivating mixtures, as for example barium carbonate or oxide, aquantity of finely divided conducting material such as carbon or one ofthe metals iron, platinum, tungsten, palladium, molybdenum and tantalum.These are added in relatively large quantities, in some cases as much as50 to by weight of the solid content of the emission material. Forlighter materials the percentage may be much smaller. Thus, in the caseof iron, 25% by weight will produce workable results. In the case ofplatinum, good results were obtained with relative proportions of A; ofa milligram of platinum to 3% milligrams of the activating material.

The mixture may be applied by the simple process of dipping the barecathode coil into the aforementioned mixture, a suitable binder, asnitrocellulose, being used to insure adherence to the coil. Good resultshave also been obtained by applying the conducting mixture as a separatecoating after the application of the emitting material. It may also beapplied in the form of a compound which is conducting in itself, orwhich breaks down under the influence of heat or other conditions togive the free metal or other conducting material. Thus, platinum hasbeen satisfactorily applied in the form of platinum chloride, whichbreaks down under the influence of heat to give a coating of freeplatinum on the cathode surface or mixed with the activating material.

I have found that the best results have been obtained by using anon-oxidizable or difficultly oxidizable material. Particles which areoxidizable apparently may become in time non-conducting throughoxidation by water vapor in the tube, or by the oxygen from the oxidesof the coating itself. If this occurs their effectiveness is generallydestroyed. Thus far, the best results coiled coil 2. It maybe formed oftungsten,

or any of the other materials common in the art. As shown, the coil 2 iscoated with the quantity 3 of activating material and conductingparticles already described.

The improved cathode may be used to ad vantage in any of the dischargelamps now cornmon in the art, such as those incorporating a gaseousatmosphere comprising a metallic vapor at pressures of the order of 10microns and. up to several millimeters of a starting gas within anelongated envelope having an activated electrode at each end. Such lampsare described in the U ted States Patent 2,182,732 to Meyer et al. Itis, of course, understood that in such lamps the ac ivated electrode ateach end of the tube act's lternately as cathode and anode. By means of.the described processes, I have been able to construct discharge lampsof this type having a' very substantial, useful life, which is free ofdisturbances creating radio interference. In one case, the radiointerference was reduced to &0 of" its usual value. My methodisobviously very simple and economical. It requires no substantial changesin the manufacturing methods; the lamp ruggedness is not afiected and itis not necessary to make any changes in the auxiliary equipment of thelamp.

It will be understood that my invention is not to be limited by anytheory of operation hereinbefore recited. Further, it will be understoodthat many modificationsin the relative percentages of the conductingmaterials and in the manner of applying them to the cathode surface willoccur to those skilled in the art to which it appertains. All suchmodifications I aim to include within the scope of the appended claim.

WhatI claim as new and. desire to secure by Letters Patent of the UnitedStates is:

In an alternating current low pressure gaseous electric arc dischargedevice of the type containing an electrode which serves alternately ascathode and anode and comprises a coiled wire filament having anappreciable voltage drop between its ends and an electron emissivecoating of alkaline earth oxide thereon and which, during operation,normally creates disturbances interfering with radios in its vicinity,means for materially suppressing the radio interference comprisingparticles of free metal selected from the group consisting of platinumand palladium distributed throughout the emissive coating and providinga good conducting surface over the electrode.

JOHN O. AICI-IER.

